This invention relates to loudspeakers and stereophonic systems.
As opposed to a point, or spherical acoustic source, a line, or cylindrical acoustic source positioned vertically in a room emits only a horizontal sound waveform. Thus, if the cylindrical source extends substantially from the floor of the room to the ceiling, there is no vertical energy component which might result in reflection of the sound wave at the floor or ceiling. As a result, the waveform that spreads into the room has a more uniform, intense pattern than that emitted from a spherical source, especially as the distance between the source and a listener increases.
The foregoing considerations were incorporated into U.S. Pat. No. 3,980,829 issued to the applicant herein. The disclosure of that patent is incorporated herein by reference and includes a loudspeaker and stereophonic system employing a semi-cylindrical waveform generator. The waveform generator of that patent comprises an elongated driver means, such as a planar sound transducer, capable of generating frequencies through substantially the entire audible range, which emits sound through an acoustical lens structure. The waveform generator and the lens employed therein have a vertical extent approximating a floor-to-ceiling height, thereby spanning the level of seated and standing listeners. The vertically elongated lens structure facilitates the generation of a uniform cylindrical sound wave that includes substantially the entire audible frequency range.
The acoustical lens structure of U.S. Pat. No. 3,980,829 comprises a series of walls which are straight in the vertical direction but are spaced apart and curved in accordance with a special pattern in the horizontal direction to define a series of channels. With respect to the special pattern, the walls simultaneously curve together to a narrow throat region and then simultaneously diverge to an outlet aperture.
The above described lens structure employed for generating the highly desirable, uniform, cylindrical sound wave uses considerable power to produce uniform acoustical loudness, or volume, through substantially the entire audible acoustic range. More specifically, a disproportionate amount of power is used to squeeze the lower frequencies through the narrow throat region of the lens structure, particularly when the driver is an electrostatic transducer.
One alternative to this power, or volume, constraint is to enlarge the throat region of the lens structure. However, this would necessitate considerable expansion of the speaker cabinet, which would generally be undesirable to consumers planning to use the speakers in relatively small rooms.
A second alternative is to employ a separate unit, termed a "sub-woofer", to generate the lowest audible frequencies below a cross-over frequency. A primary driver or generator would simultaneously generate all audible frequencies above the cross-over frequency. Hence, the lens structure and cabinet housing the primary driver and the lens could remain conveniently compact.
When a sub-woofer unit is used in conjunction with the primary driver as suggested above, a question arises regarding the physical placement in the room of the sub-woofer unit. When the cross-over frequencies are low, it may be possible (although not necessarily desirable) to locate the sub-woofer unit fairly distant from the primary generator. At these very low frequencies a listener in the room is not quite as sensitive to the time delay arising from the differing path lengths travelled by the low and high frequencies which emanate from the respective distantly separated sources. However, when desiring to utilize less operating power, and accordingly desiring to have a higher cross-over frequency, the human ear becomes acutely aware of the time delay of the shorter wavelengths in the vicinity of the cross-over frequency. The time delay results in confusion which can be both unpleasant and fatiguing.
Numerous conventional stereo systems house both a tweeter and a woofer in a speaker cabinet. However, these systems cannot adequately address the time delay problem since, having two discrete sources neither of which generates a cylindrical wavefront, either an off-axis vertical or horizontal delay degrades the quality of the sound.
Generation of low frequencies generally, either by a single woofer or a single primary driver, further involves standing wave complications, especially in a relatively small room. At these low frequencies the room dimensions are typically one or two standing-wave wavelengths. As a result, there are very broad low-frequency amplitude variations throughout the room. For example, in areas in the room where standing wave nulls occur, the amplitude, or volume, may be diminished by as much as 20 dB or even 30 dB.
The low frequency standing wave problem is not remedied in the conventional stereo system which typically employs two cabinets. In such systems the speakers are generally located horizontally symmetric with respect to a listener region since it is desirable that the high frequencies be symmetric. However, horizontal symmetric placement of the cabinets and speakers contained therein produces the same low frequency amplitude--probably diminished--at the listener region which is horizontally equidistant from each speaker cabinet.
In view of the foregoing, an object of the invention is to provide a loudspeaker which produces a uniform low frequency response which retains its time coherence with the cylindrical waveform.
Practically all loudspeaker structures give rise to a back wall reflection. That is, some of the energy generated by the driving transducer travels backward with respect to the transducer rather than forward through a speaker opening. Thus, backward-moving energy is reflected from the back wall of the speaker cabinet so that it travels back through the virtually transparent transducer and interfers with the forward moving energy wave, thus degrading the sound quality. The back wall reflection is particularly egregious in loudspeakers having a flat, or planar, back wall, since the backward energy is reflected at 180.degree. to its incident angle and produces a planar wavefront which is more destructive since it is in uniform phase with respect to the direction of the forward moving sound. In this respect, an advantage of the structure about to be described is the reduction of the back wall reflection problems in a loudspeaker.
Most loudspeakers also suffer from sound degradation caused by diffraction effects as the sound wave washers out over the edges of the cabinet containing the speaker. This diffraction, occuring at the edges of the cabinet, causes further interference with the forward-moving waves, thereby tending to diminish sound uniformity and clarity in the listener region. An advantage of the structure about to be described, therefore, is the reduction of diffractive effects associated with a loudspeaker.